I am having a similar problem. I have shocked the pool and it stays free of yellowish brown powder which collects in the same spots of the liner each time until I turn on the filtration. (It poofs when swept and vaccuums easily). I have shocked and treated with yellow rid. The pool water is perfectly clear and the pool floor will stay clear for days UNTIL I turn the filtration system on. I too believe it is coming from jets. I have backwashed and rinsed and poured in the recommended amt. of filter cleaner. Could the filter sand need to be changed. If this residue is in the pipes how do I push it out?